HYDROGEN ION CONCENTRATION 15 



phenomena are markedly influenced by the concentration of hydrogen ions in 

 the medium in which they occur. For many purposes therefore it is more 

 important to have some sort of a measuring stick of the concentration of 

 hydrogen ions present in a solution than of the total acidity of the solution. 



Total acidity, as already noted, is customarily expressed in terms of 

 normalit}'. It is also entirely possible to use the normal system for expressing 

 the concentration of hydrogen ions. In a normal solution of hydrochloric 

 acid, for example, about 78 per cent of the molecules are dissociated. The 

 term normal as used in the preceding sentence refers to total acidity, that is, 

 to all the ionizable hydrogen whether actually present as ions or combined 

 with anions in the form of molecules. In terms of the hydrogen ions present, 

 however, such a solution is only 0.78 N. The term normal as used in this 

 latter sense refers only to the ionized hydrogen. We may speak therefore of a 

 normal solution of hydrogen ions as well as of a normal solution of an acid. 



Since no acid is ever completely dissociated, a normal solution of any 

 acid will always be less than normal when its concentration is expressed in 

 terms of the hydrogen ions present. In order to prepare a normal solution of 

 hydrogen ions it is necessary to make up a solution which is more than normal 

 in terms of total acidity. Such a solution must be of the precise strength and 

 degree of dissociation that exactly 1.008 g. of the ionizable hydrogen present 

 are actually in the dissociated form — as ions — per liter of the solution. 



Although hydrogen ion concentration can be readily expressed in terms of 

 normalities, in actual practice this system is not generally used because it is 

 apt to prove cumbersome, especially when it is necessary to refer to the very 

 small concentrations of hydrogen ions usually dealt with in biological prob- 

 lems. The hydrogen ion concentration of a solution is now quite generally 

 defined in terms of its pH value. The pH value bears a simple mathematical 

 relation to the hydrogen ion concentration of a solution in terms of its nor- 

 mality. Because of the practically universal acceptance of this system it is 

 necessary to understand the significance of the term pH and its relation to 

 hydrogen ion concentration expressed in terms of normality. 



The relation between pH and hydrogen ion concentration in terms of 

 normality is a logarithmic one (Table 3). The pH of a normal solution of 

 hydrogen ions is o, of a O.i N solution i, of a O.Oi N solution 2, etc. Zero is 

 the logarithm of i, i is the logarithm of 10, 2 is the logarithm of 100, etc. 

 The pH value for any solution is the negative of the logarithm of the hydro- 

 gen ion concentration in terms of normality. It may also be defined as the 

 logarithm of the number of liters of solution that contains one gram atomic 

 weight of hydrogen ions. 



All aqueous solutions as well as pure water contain hydrogen ions in some 



